December 23, 2010

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Revisiting the 1919-1920 Mauna Iki eruption

Fast-moving lava flows erupted from Mauna Iki were hand-colored in this black-and-white photo taken on May 17, 1920. Historic photo courtesy of Roger and Barbara Myers.

Shortly after dark on December 16, 1919, a bright meteor streaked over Kīlauea like a fireball and exploded southwest of the summit. The concussion—a portent of things to come on the volcano's southwest rift zone—was heard throughout most of East Hawai`i.

The year 1919 was a lively time at Kīlauea. Halema`uma`u was a lava shield—not the large crater we see today—and formed a high point on Kīlauea's caldera floor. In early July, the lava column began rising slowly, marking an increase in activity, and the Halema`uma`u shield was soon topped by a handful of overflowing lava lakes. Lava, erupting from the north flank of the shield, swept around the eastern caldera floor below today's location of the Volcano House Hotel and pooled at the base of Byron Ledge.

This activity continued through the autumn, with lava fountains on the lava lakes, skylights along a lava tube, and active lava flows on the caldera floor. Kīlauea's incandescence illuminated the night sky. But this ended on the morning on November 28, when the lava column dropped more than 180 m (600 ft) in just a few hours. The resulting crater was 365 m (1,200 ft) across, with nearly vertical walls and a small lava lake churning at its bottom.

Recovery was quick, though, and a great flood of lava unlike any previously described at Kīlauea began to refill the crater at a rate of about 12 m (40 ft) per day. By December 15, the lava lake was a mere 6 m (20 ft) below the rim of the new crater, and its surface roiled violently with thousands of small dome fountains. Southwest of Halema`uma`u, the caldera floor began to heave and groan and, just before noon, ripped open in a long fissure. Lava poured through the crack and spread quickly across the southwestern caldera floor.

The next evening, as the meteor flashed overhead, the lava lake began to rise again. The rumble of lava fountains was clearly heard by visitors on Uwekahuna Bluff, where the Hawaiian Volcano Observatory now sits, and the caldera walls were lit by the lava lakes' brilliant glow. Unbeknownst to them, lava was pushing its way underground toward the southwest.

On December 21, upon seeing smoke in the distance, scientists journeyed into the Ka`u Desert along Kīlauea's southwest rift zone. There they discovered that lava from the summit had traveled even farther down the rift. Lava could be seen deep in cracks and was found welling from the ground 6.5 km (4 miles) from the caldera. These flows, however, were short-lived, dying out by the next day.

The main shield complex—the one nearest the caldera—burst open on December 30, releasing a torrent of lava that fed a fast-moving 'a'ā flow. The more distant fissure was inactive by early January 1920, but the main shield, which became known as Mauna Iki (Little Mountain), continued to grow and feed lava to the advancing 'a'ā flow. This flow traveled downslope to the southwest, eventually reaching within 6.5 km (4 miles) of the coast before stopping on January 12.

Mauna Iki continued to erupt and, by the end of January, the shield was about 3 km (2 miles) long and 30 m (100 ft) high. Over the following months, the lava ponds atop Mauna Iki overflowed repeatedly, and spiny 'a'ā flows seeped from its flanks. This activity persisted until mid-June, when the Mauna Iki eruption began to wane simultaneously with the lowering of the lava column at Halema`uma`u. By July 21, pits had begun to form on top of Mauna Iki as lava withdrew. No eruptive activity was seen after August 15. The lava lake at Halema`uma`u began to rise again shortly afterward, but the southwest rift eruption did not resume. After eight months of activity, Mauna Iki was dead.

Mauna Iki remains a prominent landmark on Kīlauea's southwest rift and can be reached easily by hiking the Ka`u Desert Trail in Hawai`i Volcanoes National Park. What better way to appreciate a lava shield created 80 years ago than to stand at its summit?

Kīlauea Activity Update

Lava flows have been active on the pali and coastal plain west of Kalapana over the past week. Early in the week, the easternmost of the active flow lobes advanced to within about 500 m (550 yards) of the County lava viewing area, but this flow had stalled by Thursday, December 23. A new breakout at the top of the pali (1,200-ft elevation level) was first observed on Wednesday, December 22, and this new flow appears to be robbing the lava supply from the flows on the coastal plain. The ocean entry remains inactive. Lava flows from an active vent in the northwest corner of Kīlauea crater continue to cover more of the western portion of the crater floor.

At Kīlauea's summit, the circulating lava lake in the collapse pit deep within the floor of Halema`uma`u Crater has been visible via Webcam throughout the past week. The circulation pattern was interrupted sporadically by abrupt increases in the height of the lava surface. These periods of high lava level have been short-lived, lasting up to several hours, and each ended with a sudden drop of the lava surface back to its previous level. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

Two earthquakes beneath the Hawai`i Islands were reported felt during the past week. A magnitude-3.7 earthquake occurred at 10:11 a.m. HST on Saturday, December 18, 2010, and was located 20 km (13 miles) southwest of Olowalu, Maui, at a depth of 28 km (17 miles). A magnitude-3.5 earthquake occurred at 9:30 p.m. on Monday, December 20, and was located 35 km (22 miles) north of Kapalua, Maui, at a depth of 32 km (20 miles).